Space Charge Accumulation at Material Interfaces in HVDC Cable Insulation Part I—Experimental Study and Charge Injection Hypothesis

On-site installation of accessories on extruded polymeric high voltage cables in a common practice. The procedure requires the shaping of the physical interface between the cable insulation surface and the pre-molded accessory body. On such interfaces, rough surfaces should be avoided in order to limit space charge accumulation in the insulation, which affects the cable performance by reducing insulation life-time, creating conditions for local field enhancement, and, respectively, the formation of possible breakdown path e.g. by electrical treeing. Space charge measurements on cable insulation peelings were undertaken to assess the space charge injection and accumulation on interfaces with varying degrees of surface roughness in order to improve understanding on this subject. The results of the measurements confirm the hypothesis regarding the enhancement of charge injection from rough surfaces when electric field strength exceeds a certain level. The accumulated charge density in the material is shown to strongly depend on the field strength and temperature in both polarization and subsequent depolarization measurements. These results emphasize that a bipolar charge transport model that incorporates field and temperature dependencies of charge injection, trapping, detrapping, and recombination processes needs to be adopted to accurately describe the observed electric conduction phenomena.